Experimental Research On Mechanical Properties And Formality Of Medium Manganese Steel

With the development of era, more attention has paid on the safety index and lightweight index. Currently,70% structure of the car is composed of steel. In other words if there are not high quality steels, there are not high quality cars. The third generation automobile steels’strong plastic product is between 20%-50%, the steels have both high strength and high plasticity which are very helpful to improve the passive safety of automobile. Four kinds of strengthening mechanism can improve the strength of materials, but grain refinement is the only way to improve the strength and the plastic at the same time. Therefore ultra-fine grain steels are the optimal choice to satisfy the vehicle, also are the core of the future.Currently, the study of the third generation steel concentrates on the metallurgical process and cold rolling of steel plate, galvanized technology. The study of hot/warm forming technology parameters is still a blank. This article explores ultra-fine grain martensite TG steel’s basic material parameters and compared with the first generation high strength 22MnB5 steel; the influence of hot/warm forming process parameters on the properties of samples such as heating temperature, holding time, the effect of quenching temperature. Take the tensile strength, elongation, material hardness as evaluation index, design L16 (45) orthogonal test table to carry out the optimization analysis.1) The tensile strength of TG steel is 726.3 MPa, strong plastic product is 31.09GPa% due to grain refinement effect; the fracture morphology at high temperature are the big, deep and uniform dimples which have good abilities to absorb energy. Manganese element can effectively restrain pearl nuclear, and improve the stability of austenite, even under the condition of air cooling martensite microstructure can be obtained. TG steel have good stamping formability and the hardening index is 0.547. The coefficient of normal anisotropy vary little due to the ultra-fine grain effect. Thickness of the decarburization layer is only about 0.2% of the plate thickness, which means the material has high effective thickness and the quenching performance is good. 2) The orthogonal experiment results show that the quenching temperature is the significant factors influencing the material hardness, elongation parameters and the heating temperature is the significant factors influencing the tensile strength. By comprehensive balance method and comprehensive evaluation method the optimal warm forming process are:sheet metal heated to 840℃, preservation 4 min, quenched at 500℃. There are no strict requirements on cooling temperature because under the condition of die cooling commonly can get tight ultra-fine grain martensite structure.3) The traditional hot forming parts often crack under complex stress at stamping progress. The phase transformation driving force is obviously different leads to a uneven distribution of martensite phase transformation. Using the optimized process parameters to make a car’s B-pillar, cracking phenomenon does not exist on the macro. The hardness is around 450 HV, elongation is about 11.5%, which are significantly higher than the traditional boron steel sample (about 8%). During the observation of microscopic metallographic morphology, ultra-fine grain martensite distributes uniform and the differences are not obvious. The third generation of steel has wide prospect of market and will make an outstanding contribution to improve auto safety performance.The exploration of this paper provides optical hot/warm forming technology parameters of the ultra-fine grain martensite TG steel for the third generation and also provides the theoretical basis and practice guidance which contributes a lot on the effective application of the TG steel in the future.